GROL Radio Wave Propagation 3 — Questions and Answers
Question 1: What is tropospheric ducting in VHF/UHF propagation?
- Refraction of signals through the ionosphere at VHF frequencies
- Trapping of radio waves in a layer of the atmosphere due to a temperature inversion (Correct answer)
- Diffraction of signals around mountain obstacles
- Reflection of signals off the surface of the ocean
Correct answer: Trapping of radio waves in a layer of the atmosphere due to a temperature inversion
Tropospheric ducting occurs when a temperature inversion creates a waveguide-like layer in the lower atmosphere, trapping VHF/UHF signals and allowing them to travel far beyond normal line-of-sight distances.
Question 2: At what approximate frequency range does ground-wave propagation become most effective for ship-to-shore communications?
- 3–30 MHz (HF)
- 300 kHz – 3 MHz (MF) (Correct answer)
- 30–300 MHz (VHF)
- 3–30 GHz (SHF)
Correct answer: 300 kHz – 3 MHz (MF)
Medium frequency (MF) ground waves hug the Earth's surface and can propagate hundreds of miles over seawater, making MF ideal for maritime medium-range communications.
Question 3: What is 'sporadic E' propagation?
- Steady E-layer propagation that occurs only in summer evenings
- Irregular, intense patches of ionization in the E layer enabling unexpected long-distance VHF contacts (Correct answer)
- Ground-wave enhancement caused by salt-water paths
- Tropospheric bending that affects E-layer absorption
Correct answer: Irregular, intense patches of ionization in the E layer enabling unexpected long-distance VHF contacts
Sporadic E (Es) consists of localized, dense clouds of ionization in the E layer that appear unpredictably and can reflect signals at frequencies up to 150 MHz or higher over distances of 1,000–2,500 km.
Question 4: How does seawater affect ground-wave propagation compared to dry land?
- Seawater greatly attenuates ground-wave signals
- Seawater is a much better conductor and dramatically reduces ground-wave attenuation (Correct answer)
- Seawater and dry land have identical effects on ground waves
- Seawater causes ground waves to be refracted into the ionosphere
Correct answer: Seawater is a much better conductor and dramatically reduces ground-wave attenuation
Seawater's high conductivity (approximately 5 S/m) dramatically reduces ground-wave attenuation, allowing MF signals to propagate many hundreds of miles over ocean paths.
Question 5: What is 'multipath propagation' and what problem does it cause?
- Signals traveling multiple hops via the ionosphere, causing no interference
- Two or more signal paths arriving at slightly different times, causing fading or distortion (Correct answer)
- Ground-wave signals bouncing off hills, increasing signal strength
- Tropospheric ducting extending range beyond the horizon
Correct answer: Two or more signal paths arriving at slightly different times, causing fading or distortion
Multipath propagation occurs when signals arrive via different paths with different delays, and the resulting phase differences can cause signal cancellation (fading) or intersymbol interference in digital systems.
Question 6: What is 'knife-edge diffraction' in radio propagation?
- Scattering of signals by atmospheric moisture
- Bending of radio waves around a sharp obstruction such as a mountain ridge (Correct answer)
- Absorption of signals by knife-like antenna elements
- Reflection of signals off metallic rooftops
Correct answer: Bending of radio waves around a sharp obstruction such as a mountain ridge
Knife-edge diffraction is the bending of radio waves around a sharp-edged obstacle like a ridgeline, enabling some signal energy to reach into shadowed areas beyond the obstacle.
Question 7: What happens to radio wave polarization as an HF sky-wave signal passes through the ionosphere?
- Polarization is preserved perfectly by the ionosphere
- The signal's polarization rotates due to Faraday rotation caused by the Earth's magnetic field (Correct answer)
- The ionosphere converts all signals to circular polarization
- Polarization shifts only for frequencies above the MUF
Correct answer: The signal's polarization rotates due to Faraday rotation caused by the Earth's magnetic field
The Earth's magnetic field causes the plane of polarization of HF signals to rotate as they pass through the ionosphere, a phenomenon called Faraday rotation, which means the received polarization differs from the transmitted polarization.
GROL Radio Wave Propagation 3
What is tropospheric ducting in VHF/UHF propagation?